For the purpose of generating an analog telephone ringing signal, there is required a sinewave signal that has a low frequency in the range of 16-60 Hz. In the ringing mode, the actual ringing signal is generated between about 20% and 50% of the time, depending on each individual specification. For instance, a ringing signal will sound for one second, followed by three seconds of silence. In the case of certain short line applications?, where only one to two, or more, analog lines are found, for instance ISDN, some manufacturers use a calling subscriber line circuit referred to as a Subscriber Line Interface Circuit (SLIC). This circuit sends a ringing signal to a telephone, without the use of an external ringing generator and ringing relay. In this construction, the SLIC functions as a path amplifier which is supplied with a sinus signal. The ringing signal is generated by some form of oscillator, e.g. a "Wiener bridge oscillator".
The risk of interference can be reduced and the power consumption of the SLIC kept low when the SLIC is not in a ringing Tnode, by switching-off the oscillator and then starting the oscillator when necessary. Unfortunately, this cannot always be easily achieved since the time lapse from the moment of applying the voltage to the moment at which the sinewave ringing signal is fully developed is very long in comparison with the period time of the ringing signal in the case of that type of precision oscillator which is required to fulfil the requirement of producing a ringing signal of low distortion. It is necessary to start the oscillator at least one second before a fully developed output signal has been obtained, which renders this solution unrealistic in practice. The start time can be shortened, although at the cost of increased distortion--meaning that it is unrealistic in practice.